This application is also related to application Ser. No. 09/295,102 filed on the same date as this application, entitled xe2x80x9cInsert for Use in Conjoining Tubular End Portion,xe2x80x9d by Michael W. Johnson and Jeffrey J. McKenzie. These applications are incorporated by reference herein.
This invention relates to an apparatus and method for welding thermoplastic tubular end portions. More particularly the invention relates to an apparatus and method particularly suitable for welding PFA (perfluoroalkoxy).
Various means have been known for welding together the ends of thermoplastic pipes or tubes. U.S. Pat. No. 3,013,925 discloses inserting a heated platen between the ends of lined pipes and causing the pipes to bear against the heated platen to fuse the plastic after which, the platen is removed and the pipes are pressed together under pressure until welding has occurred and the thermoplastic material hardens as it cools. U.S. Pat. Nos. 5,037,500; 4,792,374; and 5,484,506 all disclose exterior conduction heaters in which abutted tubular ends are peripherally heated to weld the ends together.
U.S. Pat. No. 4,929,293 to Osgar utilizes the placement of an infrared heating plate in-between and not contacting the tubular end pieces to be joined. The infrared heating plate is removed and the tube joints are then engaged together to create the weld. Although such a procedure provides a high integrity weld on PFA and other melt processable plastics, such a weld typically leaves a deformation or a bead on the inner and/or outer surfaces of the joined tubular end portions.
In many applications it is unacceptable to have any deformations in the joined tube particularly on the interior surface. In sanitary systems used in the food processing and pharmaceutical industries the standards of the U.S. Code of Regulations, 7 C.F.R. ?58.128 require a conduit be smooth, permit laminar flow of fluids and be free of discontinuities that could trap particulate matter.
Traditionally, stainless steel tubing and pipe have been used in sanitary systems. However, due to corrosion, expense and other problems, plastic pipe and tubing are now seeing more use in such systems and PFA has the particular advantages of its high chemical inertness and resistance to the high temperature cleaning and sanitizing temperatures.
Applicants are not aware of any conduction heat weld system that has peripheral contact with abutted tubular ends has been shown to be suitable for welding PFA. This is due to the difficulties associated with the higher melt temperatures of PFA (approximately 590 F.) and the characteristics of PFA. For example, melted PFA sticks to many different materials that are commonly used in welding devices. Such sticking can render the completed weld defective and can cause significant operational problems with the weld equipment. Additionally, heating and melting of PFA produces fluorine gas which is highly corrosive to conventional materials utilized in fusion welding equipment. For example standard stainless steel quickly pits when exposed to fluorine gas. Additionally, PFA when melted expands and if constrained can develop extremely high pressures causing the PFA to leach out of the weld are into crevices or other undesired locations.
Also problematic with welding higher temperature melt processable plastics, such as PFA, is the extended cycle time for a weld. The typical generic steps for a weld are: 1) position the tubular end portions to be welded in the weld head; 2) close the weld head; 3) warm up the weld head and heater portion; 4) melt and weld the end portions; and 5) allow the welded part to cool; and 6) remove the welded component from the weld head. Conventional conduction heating heads utilize a pair of integral heater portions each of which extend longitudinally down the abutted tubular end portions and totally enclose the melt portion of the end portions. Such conventional weld portions are formed of stainless steel which has a relatively high thermal conductivity. This makes it difficult to isolate and minimize the melt zone which in turn effects the length of the cycle time. Moreover, in conventional weld beads and weld apparatus essentially all of the components are made of metal. This increases the warm-up period and the cool down period. With the elevated temperatures associated with PFA, these problems are exacerbated.
Attempts have been made to reduce the cycle time of welding thermoplastic tubular end portions such as by providing heat sink arrangements and forced cooling. The high thermal conductivity of metals and particularly stainless steel minimizes the effectiveness of such measures and ancillary cooling equipment adds cost, complexity, and maintenance problems.
A welding apparatus, system, and methodology is needed for creating beadless welds in plastic tubular end portions, particularly PFA, by conduction heating. Moreover, there is a need for reducing cycle time in welding thermoplastics.
When tubular end portions of PFA are heated to beyond their melt point a minimal, although significant, amount of expansion of the molten PFA material occurs. When the tubular end portions being welded are tightly constrained this expansion causes the molten PFA to leach into any crevices or imperfections in the weld head and/or mandrel. Moreover, if the tubular end portions are secured in place after the weld is cooled there will typically be a narrowing of the material at the weld site. This can cause an hourglass shape and/or a reduced wall thickness at the juncture. Where the PFA has leached into minute crevices or imperfections, flashing may exist on the exterior or interior surfaces of the tube when the weld has cooled and is removed from the weld head. These would typically necessitate manual removal and in severe cases may make the welded components unusable in particular applications. Thus, a need exists for minimizing or reducing the currents of flashing, the reduced diameter at the weld juncture, and reduced wall thickness at the weld juncture due to the expansion and contraction of PFA during the weld operation.
A conduction welding apparatus and method for bonding abutted thermal plastic tubular end portions is particularly suitable for welding PFA. In a preferred embodiment, a folding weld head embraces the abutted the tubular end portions to be welded, each tubular end portion including a flange spaced from the surfaces to be joined. The weld head includes means for securing the flanges within the weld head and further include bias means. The bias means provide an inward axial bias on the adjoined tubular end portions when the tubular end portions are displaced axially outward from the weld head due to the expansion of the plastic during the weld process. The bias means in a preferred embodiment is substantially inoperative prior to the weld and provides bias, or a substantial increase in bias, when the flanges are displaced due to the expansion of the molten PFA during the weld process. In a preferred embodiment, the securing means comprises a pair of clamps which each attach to and encompass the respective tubular end portions at the end portion flanges. The tubing clamps fit into recesses in the weld head. In such a preferred embodiment the bias means is provided by a spring loaded plate which is deflectable in an axial direction outward from the weld head. The invention also includes a process for accomplishing a weld with the described apparatus.
In a preferred embodiment a composite heater portion provides a sharp temperature gradient from the weld juncture outward. The abutted tubular end portions are positioned at a central heated section of relatively narrow thickness in the axial direction with respect to the tubular end portions. The central heated section is preferably formed of two half sections each with a semicylindrical cavity. The two semicylindrical cavities form a cylindrical cavity for embracing the abutted end portions and may be hinged together. Sandwiched around the central heated section are a pair of isolating secondary sections layered adjacent to the central section. The secondary sections will similarly have semicylindrical cavities for receiving the tubular end portions.
The central section is preferably comprised of a material of a substantially higher heat conductivity than the secondary sections. This facilitates rapid heating of the central heated section and slower heating of the secondary sections. The sandwiched sections may be suitably bonded or mechanically secured together. In a preferred embodiment, at least part of the tubing contact portion of the heater portion, such as the isolating secondary sections are comprised of a plastic with high temperature resistance. In a preferred embodiment, the central at the tubing contact portion. Alternatively, ceramic materials may be used at the secondary sections. The central section and outer sections may be of generally the same material with different heat conductive characteristics; for example similar ceramic materials with a large difference in thermal conductivities or may be of very different materials such as copper or stainless steel for the central section and plastic for the secondary sections.
In a preferred embodiment particularly useful for welding PFA, the abutted tubular end portions may be wrapped with an impervious sheet material such as a polyamide film. An object and advantage of the use of the wrap is that the weld head is isolated from the weld without loss of any significant heat transfer to the weld. Thus if the weld head is comprised of any materials susceptible to corrosion from the fluorine gas emitted by the melted PFA, such material are isolated from the gas. Moreover the sheet material operates to prevent the PFA from migrating out of the weld zone, for example, into the junctures of the heater portion layers. A further object and advantage of the use of the sheet material is that the weld has a smooth exterior finish.
The heater head preferably comprises a body with a base portion with a first semicylindrical cavity and a folding portion with a cooperating second semicylindrical cavity to embrace the abutted tubular end portions. A heater portion is secured in each of the respective body portions and is sized to provide snug engagement with the exterior of the tubular end portions to provide heat to the juncture by conduction. The heater head has a pair of slots adjacent to the isolating secondary sections for receiving a pair of clamps which are configured to grasp the tubular end portions to be joined. The clamps are configured to either grasp the exterior cylindrical surface of the tubing portion or a flange that is integral with the tubular end portions of certain fittings/components. The clamps thus provide flexibility of joining flanged tubular end portion to flanged tubular end portion, unflanged portions to flanged portions, and unflanged portions to unflanged portions. Moreover, each of the clamps provide an inward circumferential radial pressure on each of the end portions which is effective to hold the insert in place and maintain the centering within the end portions at the weld juncture during handling, placement in the weld head, and welding. To accomplish a weld the clamps are applied to the tubular end portions to be welded, an insert is placed in the tubular end portions and the end portions with attached clamps are then inserted into the heater head. The clamps are preferably formed of a heat resistant plastic such as a polyetheretherketone (PEEK). Such a material has a very low thermal conductivity and much lower than metals conventionally used in weld heads.
The weld cycle of the weld system is preferably controlled by conventional automated means by a controller. Particularly for welding PFA, in order to minimize environmental effects such as the ambient temperature, the heater element may be initially heated and held to a first temperature below the weld temperature for a period of several minutes, the temperature is then lowered to a lesser amount for several minutes and then is rapidly ramped up to the weld temperature for sufficient time to accomplish the weld. The temperature is then ramped down and the joint and weld head are allowed to cool before removal of the welded component. The minimization of the use of metal in the weld head for the heater portion and the clamps allows the desired temperature to be approached quicker and allows the weld head and weld to cool quicker. The ramping down and holding the power level for several minutes before powering up to the weld temperature appears to allow the weld head temperatures to stabilize sufficiently under varying ambient conditions to provide consistently good welds.
In a preferred embodiment of the invention, several different weld heads which are each configured for a specific tubing size are receivable in a receiver portion on a bench top base. This bench top base has adjustable universal tubing supports that do not require any additional clamping of the tubing members into the support nor do the supports have to be adjusted in any way to provide proper height and alignment of the tubing. Each universal support is spaced from the receiver and the weld head for the bench top base. The various heater heads, each of which is adapted for a specific tubing size, are each placeable in the receiver on the base. Each heater head for a specific size tubing has a configuration to position the lowest portion of the outside diameter of tubing clamped in the head at the same relative elevation as the lowest point on the support surfaces of the universal tubing support.
In a preferred embodiment of the invention, several different weld heads which are each configured for a specific tubing size are receivable in a receiver portion on a bench top base. This bench top base has adjustable universal tubing supports that do not require any additional clamping of the tubing members into the support nor do the supports have to be adjusted in any way to provide proper height and alignment of the tubing. Each universal support is spaced from the receiver and the weld head for the bench top base. The various heater heads, each of which is adapted for a specific tubing size, are each placeable in the receiver on the base. Each heater head for a specific size tubing has a configuration to position the lowest portion of the outside diameter of tubing clamped in the head at the same relative elevation as the lowest point on the support surfaces of the universal tubing support.
Alternatively, inserts which include the heater portion which embraces the tubing to be welded, can be sized for placing the lowest point on the outside diameter of embraced tubing at the same relative elevation as the lowest point on the support surfaces of the universal tubing support.
In a further alternative embodiment, the universal support can have a V-shape such that the supported tubing has two points of contact providing slightly more lateral stability.
Thus, an object and feature of the system is to provide a readily adaptable portable or bench top conduction welding apparatus for various sizes of tubing which requires no adjustable of or mechanical manipulation of the tubing supports when different sizes of tubing are welded.
Another object and advantage of particular embodiments of the invention is that metal to tubing contact is absolutely minimized or eliminated. This facilitates the use of less power per weld, minimizes cycle time, accelerates heating and cooling of the tubular end portions, keeps the exterior of the tubular end portions in better condition, and provides an environment very conductive to welding PFA.
A principal object and advantage of particular embodiments of the invention is that the weld zone is minimized resulting in quicker cycle times.
A feature and advantage of the invention is that the spring-loaded bias is provided only when the tubular end portions expand axially outward.
A further advantage and feature of the invention is that structural anomalies at the weld site such as narrowed wall thickness, flashing, hourglass shape can be minimized or eliminated by the invention.
Further object and advantage of the invention is that the weld head maintains its compact shape while still providing the bias during the weld process.